In the conventional art, circuit operation in integrated circuits often depends upon one or more accurate and stable voltage references. For example, numerous analog circuits, such as amplifiers, current mirrors and the like depend upon current sources that conduct stable currents. The accuracy and stability of the current sources typically depend upon the accuracy and stability of one or more reference voltages applied to the gates of transistors that provide the current sources. Other circuits, particularly those that control the switching response of digital circuits depend upon the accuracy and stability of the reference voltages to control the switching speeds, slew rates and/or the like of the circuit.
Even a relatively process/voltage/temperature (PVT) insensitive voltage reference exhibits some variations from a desired voltage. For example, a reference circuit for generating 3V may exhibit a ±60 mV variance over all PVT variations. Accordingly, adjustment of the voltage reference value may sometimes be required to compensate for fabrication process variations, voltage variations and/or temperature variations.
Referring to FIG. 1, a block diagram of a voltage trim circuit 100, in accordance with the conventional art, is shown. The input reference voltage VIN may be generated by a bandgap type reference circuit or other similar PVT insensitive reference circuit. The operational amplifier 105 compares the input reference voltage (VIN) with the feedback voltage (VFB), and depending upon the state of the switches 130–145 and the resistive values 115–125, can adjust the output reference voltage (VOUT) above or below the input reference voltage (VIN). Another voltage trim circuit is disclosed in McClure et al., U.S. Pat. No. 6,281,734 issued Aug. 28, 2001.
The conventional voltage trim circuits allow for correction of PVT induced reference voltage variations. However, the conventional voltage trim circuits may exhibit instability. Therefore, there is a continued need for an improved voltage trim circuit.